This invention relates generally to pseudomorphic high electron mobility transistors (PHEMTs) and more particularly to transistors of such type which are formed on III-V substrates.
As is known in the art, there are several types of active devices used at microwave and millimeter frequencies to provide amplification of radio frequency signals. In general, one of the more common semiconductor devices used at these frequencies is the field effect transistor, in particular, metal electrode semiconductor field effect transistors (MESFETs), high electron mobility transistors (HEMTs), and pseudomorphic high electron mobility transistors (PHEMTs). Each of these transistors is formed from Group III-V materials such as gallium arsenide (GaAs) or indium phosphide (InP). What distinguishes a HEMT from a MESFET is that in a HEMT there is a doped barrier, or donor, layer of one material and an undoped channel layer of a different material. A heterojunction is formed between the doped donor layer and the undoped channel layer. This heterojunction provides spatial separation of electrons which are injected from the doped donor layer into the undoped channel layer. Thus, electrons from the large bandgap donor layer are transferred into the narrow bandgap channel layer where they are confined to move only in a plane parallel to the heterojunction. This results in the formation of a two-dimensional electron so-called "gas". Because conduction takes place in the undoped channel, impurity scattering is reduced in this undoped layer and electron mobility is thereby enhanced compared to the doped channel structure used in MESFETs. Accordingly, HEMTs provide higher frequency operation than MESFETs.
As is also known in the art, one type of PHEMT includes a gallium arsenide substrate having formed thereon successive layers of: an undoped InGaAs channel layer; a doped barrier (donor) AlGaAs layer; and n- GaAs and n+ GaAs ohmic contact layers, as shown in FIG. 1A. A layer of photoresist is then deposited over the structure and patterned to have an aperture over a portion of the structure to expose a region where the gate electrode is to be formed. Using the patterned photoresist as an etching mask, an etch is brought into contact with the portions exposed by the aperture to successively etch through portions of the n+ GaAs and n- GaAs layers and partially into the AlGaAs layer, as shown in FIG. 1A for a wet etch and FIG. 1B for a dry etch. In either case, a relatively wide recess is formed in the n+ GaAs and n- GaAs ohmic contact layers thereby improving the breakdown voltage of the FET. The dry etch has better selectivity and less undercut than a wet etch; however, the dry etch always causes some damage on the surface layer being etched which may induce more unwanted surface states. In either the wet or dry etch process, after the wide recess is formed, the photoresist is stripped and another layer of photoresist is deposited over the structure and patterned to define the narrow gate recess and gate metalization (i.e., the gate electrode) in Schottky contact with the AlGaAs channel layer as shown in FIG. 1C (when the wet etch is used to form the wide recess), or in FIG. 1D (when the dry etch is used to form the wide recess). In either case, for the AlGaAs PHEMT shown in either FIG. 1C or 1D, this narrow recess is performed with a wet chemical etch by a timed etch which is checked by measuring the open channel current between the source S and drain D electrodes. A gate metal is then deposited over the photoresist and through the electron beam patterned aperture formed therein onto the exposed portion of the aluminum gallium arsenide layer. After the photoresist layer and extraneous metal thereon are lifted-off, the gate electrode G is formed. The resulting FET is shown in FIG. 1E for the wet etch process and FIG. 1F for the dry etch process. If, on the other hand, instead of using the AlGaAs layer, a layer of InGaP was used, the use of the second photoresist layer on an InGaP surface and the use of a wet etch are not compatible. More particularly, the wet etch used for etching InGaP are solutions containing strong acids. These strong acids cause sever undercutting in the photoresist resulting in a complete loss of the InGaP surface layer. Further, it is noted from FIGS. 1E and 1F, that there is a significant portion of the recess which is ungated, designated by U in the figures, which results from either of these processes.